1. Field of the Invention
The present invention relates to the field of wireless communications devices and networks. More specifically, the present invention relates to a system and method for synchronizing phases and frequencies of devices in multi-user, wireless communications systems.
2. Related Art
In multiuser wireless communication systems, the term “distributed beamforming” describes the situation in which two or more separate transmitters with common information work together to emulate an antenna array and to focus their bandpass transmissions toward an intended destination. Distributed beamforming has also been referred to as “collaborative beamforming” and “virtual antenna arrays,” and has also been discussed in the context of coherent cooperative transmission and cooperative multiple-input/multiple-output (MIMO) transmission. In all of these systems, the basic principle is the same: individual sources with common information transmit with phase-aligned carriers such that their bandpass transmissions combine constructively after propagation to the intended destination.
There are several advantages to distributed beamforming. By focusing the transmission toward the intended destination, for instance, less transmit power is needed to achieve a desired signal-to-noise ratio (SNR) target. This feature is particularly appealing in wireless communication systems with energy-constrained nodes, such as sensor networks. In these types of systems, nodes are typically too small to allow for the use of conventional antenna arrays. Distributed beamforming is a powerful technique that offers the potential power gains of conventional antenna arrays to wireless communication systems composed of multiple single-antenna users.
Difficulties with distributed beamforming systems relate to the alignment (synchronization) of phases and frequencies of the transmitters. This difficulty is due, in large part, to the fact that each transmitter in a distributed beamformer has an independent local oscillator which can drift in phase and frequency over time. Transmitters in a distributed beamformer therefore require some method to synchronize their carrier signals so that the bandpass transmissions arrive with reasonable phase and frequency alignment at the intended destination. A precise phase alignment is not critical for beamforming. For example, a two-antenna beamformer with a 30 degree phase offset in the received carriers only suffers a loss with respect to ideal beamforming of approximately 7% of the power in the intended direction. The power gain of a beamformer becomes a power penalty, however, when the carriers arrive at the destination with more than 90 degrees of phase offset. Therefore, there is a need for adequate carrier synchronization to ensure energy-efficient transmission to the destination and to ensure that the sources do not cancel each other's transmissions.